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Let's start by describing how the data on a freelist is laid out in memory.
This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes):
</p><pre class="programlisting">
+----------------+
| next* ---------|--+  (_S_bin[ 3 ].first[ 3 ] points here)
|                |  |
|                |  |
|                |  |
+----------------+  |
| thread_id = 3  |  |
|                |  |
|                |  |
|                |  |
+----------------+  |
| DATA           |  |  (A pointer to here is what is returned to the
|                |  |   the application when needed)
|                |  |
|                |  |
|                |  |
|                |  |
|                |  |
|                |  |
+----------------+  |
+----------------+  |
| next*          |&lt;-+  (If next == NULL it's the last one on the list)
|                |
|                |
|                |
+----------------+
| thread_id = 3  |
|                |
|                |
|                |
+----------------+
| DATA           |
|                |
|                |
|                |
|                |
|                |
|                |
|                |
+----------------+
</pre><p>
With this in mind we simplify things a bit for a while and say that there is
only one thread (a ST application). In this case all operations are made to
what is referred to as the global pool - thread id 0 (No thread may be
assigned this id since they span from 1 to _S_max_threads in a MT application).
</p><p>
When the application requests memory (calling allocate()) we first look at the
requested size and if this is &gt; _S_max_bytes we call new() directly and return.
</p><p>
If the requested size is within limits we start by finding out from which
bin we should serve this request by looking in _S_binmap.
</p><p>
A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of
this size on the freelist (0). If this is not NULL - fine, just remove the
block that _S_bin[ bin ].first[ 0 ] points to from the list,
update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data.
</p><p>
If the freelist is empty (the pointer is NULL) we must get memory from the
system and build us a freelist within this memory. All requests for new memory
is made in chunks of _S_chunk_size. Knowing the size of a block_record and
the bytes that this bin stores we then calculate how many blocks we can create
within this chunk, build the list, remove the first block, update the pointer
(_S_bin[ bin ].first[ 0 ]) and return a pointer to that blocks data.
</p><p>
Deallocation is equally simple; the pointer is casted back to a block_record
pointer, lookup which bin to use based on the size, add the block to the front
of the global freelist and update the pointer as needed
(_S_bin[ bin ].first[ 0 ]).
</p><p>
The decision to add deallocated blocks to the front of the freelist was made
after a set of performance measurements that showed that this is roughly 10%
faster than maintaining a set of "last pointers" as well.
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